Written by John Aho Monday, 05 September 2005
Installing Exhaust Manifold or Header Studs
By John Aho
Studs are designed to be fitted only "finger-tight" into their threaded hole in a casting. However, a locking type of nut (PTN, or prevailing torque nut) is often specified to go on the stud. These two facts present an inherent dilemma. The grip of the PTN on the stud can easily turn the stud when you don't want it to (how often have you removed an exhaust manifold nut and had the stud come out at the same time?)
With this in mind, the installer may be tempted to over tighten the stud into the hole, to make certain it stays put. This mistake happens a couple of ways: Either the stud gets torqued after it bottoms in the hole, or the stud's shoulder (unthreaded section in between the threaded ends) jams into the surface at the edge of the hole. Some studs have no shoulder (all thread), and some holes have no bottom (through hole). This allows for another problem: with nothing to stop it, the stud can insert too deep.
So, why is it wrong to tighten the stud into the hole? Unfortunately, I have even seen it done by supposed "professionals". The way a stud can outperform a bolt in clamping efficiency has to do with the even distribution of stress across the engaged threads. If you torque a stud into a blind hole, you lose that benefit, and concentrate stresses at the first thread in the hole. If you torque the shoulder of a stud against the edge of a threaded hole, you lose the benefit & usually distort the mating surface. It's not as if there is a suitable bearing surface to take the load. Just don't do it.
Loctite threadlocker is the answer for securing the stud (for most of our situations), but it's worthwhile to look at the design intent of the purely mechanical fit. Studs require a "fit classification" that provides a very close fit. Common American thread classes include #2 (free fit, general application) and #3 (very close fit). Classes #4 & 5 are actually interference fits, and you won't find them readily available.
Metric thread fits are classed differently, using a number to designate "tolerance grade", and a letter to indicate "fundamental deviation" (upper case letter if internal thread). When a class of fit must be provided for mating threaded parts, the tolerance symbols of the internal thread is shown first, then a forward slash, followed by the external thread's symbols (for example, 5H/4h). Basically, ISO metric "6" approximates a UNF class #2 fit. The metric range also includes 4 & 5 (closer) and 7 & 8 (looser). The letter H or h indicates deviation allowance = none.
You can get a decent "ball park" on fit class by the feel of threads engaging, but to really measure it is much harder. The major diameter (O.D.) of the stud's thread is not the primary determining factor for class of fit. The pitch diameter (an imaginary cylinder located where the thread width is equal to the space between adjacent threads), which requires a special "thread micrometer" or the 3-wire system to measure, determines the class of fit. The bottom line is you should try to only use metric steel studs designated as 4h. Good luck finding them.
The '02 parts book shows M8 x 40 studs, but the length used can vary from 38 to 45 mm depending on availability and/or thickness of the header flange. Be sure to select studs with a shoulder. Customarily, metric steel studs have their nominal length (which protrudes) less than their overall length by approximately 1.25 x diameter. This would mean about 10 mm of thread goes into the head. The last BMW studs I got had about 15 mm to thread in the head, which is even better. The blind holes on the bottom row of the exhaust side are about 20 mm deep, so you won't bottom out as long as you put in the correct (short) end. There's some decent manifold studs used by other manufacturers, if you look around. The Saturn cars use the same size, and have a feature on the outer end that is made to fit a female E-7 torx socket.
So what does all this mean to me when I've got an exhaust stud backing itself out of a buggered hole on a 2002 head? You will have to clean out the hole completely to get the Loctite compound to work properly. Getting this done on the car, with the manifold still in place, is a pain. Even with the exhaust out of the way, the top row of stud holes go right through into the head, making them very hard to get clean & dry.
To proceed: the loose studs will come right out, and you can "double-nut" to remove the rest.
- Take off the valve cover and blot away the oil around where the top four threaded holes go through.
- Carefully use a proper thread chaser to clear out any old threadlocker or other crud.
- Clear out the debris you've just shoved into the head before it washes down into the sump.
Use an aerosol can of brake cleaner to wash out the threads, use a rag pushed tight up against the inside of each through hole to prevent solvent/debris from getting inside.
Please Note: The Loctite compound will only work if you do a good job getting things clean & dry. Follow the instructions that come with the product. Try to find Loctite 262 (a bearing supply house will stock it), otherwise use the readily available 271. Again, use as directed: just one drop on each clean, dry, new stud as they go in.
After the Locktite has set, reinstall the exhaust with new gaskets, and torque the 8 new copper plated steel PTN to 30 Nm (22 ft lb).